1. Field of the Invention
The present invention relates to a liquid crystal display device such as a matrix liquid crystal display device used for various types of office automation apparatus such as a personal computer and a wordprocessor, multi-media information terminals, AV apparatus, game machines, and the like, and a driving method of such a liquid crystal display device.
2. Description of the Related Art
A simple matrix liquid crystal display device using twisted nematic (TN) liquid crystal and super twisted nematic (STN) liquid crystal which respond to an effective voltage is known. Such a simple matrix liquid crystal display device includes a liquid crystal panel having scanning electrodes and data electrodes crossing each other with liquid crystal therebetween. The simple matrix liquid crystal display device is driven by a line-sequential driving method.
In the line-sequential driving method, a scanning signal is applied to the scanning electrodes so as to sequentially select the scanning electrodes one by one. In synchronization with the selection of the scanning electrodes, a signal corresponding to display data for pixels on the selected scanning electrode is applied to the data electrodes.
In recent years, with the increasing request for display of multi-media information, the simple matrix liquid crystal display device using STN liquid crystal has been required to display video images and images for amusement. To meet this requirement, the display quality of the device should be improved.
In order to improve the display quality, the number of scanning lines of a liquid crystal panel may be increased. However, in the liquid crystal display device with high-speed response employing the aforementioned conventional line-sequential driving method, when the number of scanning lines is increased, a "frame response phenomenon" becomes significant, where the transmittance of the device does not respond to the effective voltage but to a driving waveform itself. Thus causes the transmittance to vary every frame and thus lowers the brightness of the device.
In order to overcome the above problem, the following three driving methods have been proposed.
(1) An active addressing (AA) method, where a WALSH function or the like is used as an orthogonal function. As shown in FIG. 14, a positive or negative voltage (1 or -1) obtained from this function is applied to all scanning electrodes (F1 to F16) simultaneously so that the orthogonality is established within one frame period TF, i.e., the inner product of a row vector is equal to zero. (T. J. Scheffer et al., SID '92, Digest, p. 228; Japanese Publication No. 7-120147; etc.)
(2) A sequency addressing (SAT) method, where one frame period TF is equally divided into a plurality of sub-periods, e.g., four sub-periods as shown in FIG. 15. In each sub-period, a plurality of scanning electrodes, e.g., four scanning electrodes in this illustrative example, are simultaneously selected so that the orthogonality is established within one frame period TF. (T. N. Ruckmongathan et al., Japan Display '92, Digest, p. 65; Japanese Laid-Open Publication No. 5-46127; etc.)
(3) A method (hereinbelow, referred to as a driving method 3), where, as shown in FIG. 16, the scanning electrodes are grouped into a plurality of blocks (framed portions in the figure) each composed of scanning electrodes in the quantity smaller than the total number of scanning electrodes. Each block is divided into a plurality of groups each composed of scanning electrodes in the quantity smaller than the number of scanning electrodes in each block. A selection pulse sequence in accordance with an orthogonal function is supplied to the scanning electrodes in each block (indicated by L) group by group sequentially for a divided sub-period T of one frame period TF which is a period required to display one screen. The pulse is applied every predetermined time during the divided sub-period T, while a voltage of a fixed level is applied during the period other than the selected sub-period. A voltage corresponding to the sum of products of the orthogonal function and display data is applied to the data electrodes. These operations are performed for all the blocks within one frame period TF by shifting the timing. (Japanese Laid-Open Publication No. 6-291848)
However, all the above three driving methods tend to cause troubles, such as shadowing (whitening) in a horizontal direction (column direction) of the panel due to the difference between the electrical capacitance at portions of a liquid crystal material in the ON state and that at portions thereof in the OFF state, and image doubling due to dulling of the selection pulse itself, lowering the display quality. These troubles will be described in detail with reference to FIGS. 17 and 18.
(i) FIG. 17 shows an upper-half portion of a liquid crystal panel having 640 pixels in a horizontal direction and 480 pixels in a vertical direction. A black block (shown by hatched lines) is displayed for a white background on the upper-half portion. The liquid crystal capacitances at the following positions shown in FIG. 17 can be obtained by respective expressions as follows.
Points A and C (liquid crystal capacitance of ON pixels): EQU C.sub.ON =.epsilon..sub.ON .times..epsilon..sub.0 .times.(S/d)
Point B (liquid crystal capacitance of OFF pixels): EQU C.sub.OFF =.epsilon..sub.OFF .times..epsilon..sub.0 .times.(S/d)
Row R.sub.1 crossing black block: EQU C.sub.R1 =C.sub.OFF .times.W+C.sub.ON .times.(W-w)
Row R.sub.2 running only white background: EQU C.sub.R2 =C.sub.ON .times.W
where .epsilon..sub.0 denotes the dielectric constant in vacuum; S denotes the area of one pixel; d denotes the cell thickness; .epsilon..sub.ON denotes the dielectric constant of an ON pixel; .epsilon..sub.OFF denotes the dielectric constant of an OFF pixel; w denotes the length (number of dots) in the horizontal direction of the black block; W denotes the length (number of dots) in the horizontal direction of the panel; R denotes the electrode resistance; and .tau..sub.Ri denotes the time constant of row R.sub.i (i=1, 2).
The difference between the time constant of row R.sub.1 and that of row R.sub.2 is represented by: ##EQU1##
Accordingly, as the time constant of row R.sub.2 is greater than that of row R.sub.1, the waveform of the selection pulse applied to row R.sub.2 is more dulled than that applied to row R.sub.1 as shown in FIGS. 18A and 18B, where solid lines represent the actual waveforms while the dash-dot lines represent ideal waveforms. As a result, the brightness at point A on a side of the black block is relatively higher than that at point C. This forms a band on the sides of the black block which appears brighter than the other portions of the screen, thus generating the shadowing (whitening).
(ii) Image Doubling
If the waveform at the tail of a selection pulse is dulled as shown in FIG. 18B, a portion which is not included in the current selected sub-period is also applied with the pulse. This results in the image doubling where a same image is vaguely displayed at a position shifted by the number of selected scanning electrodes.